<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <title>菜鸟教程(runoob.com)</title>
  <style>
  .flex-container {
    display: -webkit-flex;
    display: flex;
    width: 400px;
    height: 400px;
    background-color: lightgrey;
    flex-direction: column;
    justify-content: center;
    align-items: center;
  }

  .flex-item {
    margin:2px ;
    background-color: cornflowerblue;
    width: 100px;
    height: 100px;
    /*margin: 10px;*/
  }
  </style>
</head>
<body>

<div class="flex-container">
  <div class="flex-item">flex item 1</div>
  <div class="flex-item">flex item 2</div>
  <div class="flex-item">flex item 3</div>
</div>

</body>
</html>
